Why Did San Francisco’s Sea Lions Suddenly Start Attacking Swimmers?

The sun was setting on Tuesday, December 12, as Anders Jakobsson started his usual swim in San Francisco’s Aquatic Park cove. As the 50-year-old’s body began to warm up in the cold water, he felt something clamp down on his right foot. It was powerful, but not sharp. “I was in salt water and 10 minutes out from land,” he says, recalling his terror. “There’s no escaping that.” Jakobsson returned shaken, but safe, to shore.

Christian Einfeldt wasn’t as lucky. The 56-year-old likes to take a dip in the cove between Fisherman’s Wharf and Fort Mason during his lunch break—he calls the water his fountain of youth. On December 14, approaching the buoy a quarter mile from shore in an area swimmers call the Sea Lion Highway, he spotted a male sea lion closing in on him. He yelled “No!,” but it was too late. The animal sank a single tooth deep into his arm—so deep he could make out his muscle fibers. “My skin was shredded,” Einfeldt says. “I could see parts of my arm flopping around like a jellyfish.” Powered by adrenaline, he was able to push himself away from the animal and swim to a nearby boat, which brought him to Pier 45.

In the space of a single month this winter, four different swimmers reported attacks by sea lions in or near Aquatic Park. Sensing the beginning of a disturbing trend, on December 15 authorities closed the cove until they could get a handle on why the normally peaceful animals have seemingly turned against us.

Sea lion attacks are normally pretty infrequent. In part that’s because of the animals’ amiable nature—they may be the size of bears, but they have the personality of dogs—and in part it’s because their prey tends to be much smaller. (They mainly eat fish and crabs and other shellfish.) Confronted by a human intruder, most sea lions choose flight over fight. In fact, between 2006 and 2015, there were only two cases of sea lions accosting human swimmers in Aquatic Park, according to Claire Simeone, a veterinarian at the Marine Mammal Center in Sausalito. Far more common, though still unusual, are attacks by seals, whose bite can result in a nasty infection of Mycoplasma phocacerebrale, or “seal finger.”

Whether the recent spate of sea lion violence is the work of a single marauding animal or several is not clear—in every case the perpetrator slipped away. But Lynn Cullivan, a public information officer for the San Francisco Maritime National Historical Park, thinks it’s unlikely to be one deranged animal causing the ruckus. Which points to a troubling possibility: that the sudden aggression may be the result of something in the water poisoning the animals’ minds. In 2015, Simeone and two colleagues published a study on local pinniped attacks and ruled out several environmental factors: Rain patterns and water salinity, for instance, didn’t have any correlation to incidences of attacks. Further, attacking sea lions weren’t protecting their young, as the animals’ breeding grounds are far away, between the Channel Islands and Baja. And the local food supply doesn’t appear to have dried up, either.

Instead, scientists’ best guess is that the sea lion attacks might be associated with exposure to domoic acid, a neurotoxin found in California’s coastal waters that’s produced by the alga Pseudo-nitzschia. When waters warm, the frequency and intensity of blooms of the alga increase, pumping more of the toxin into the ocean. In 2016, a surge of domoic acid led state regulators to delay the opening of the commercial Dungeness crab season by months, over fears that humans and other large mammals who consumed infected crabs would suffer brain damage. As Garet Lahvis, a behavioral neuroscientist at Oregon Health & Science University, in Portland, explains, “Domoic acid has a profound effect on the brain, including, in experimental situations, aggression.” (Consider: The 1961 bird attack in Santa Cruz that inspired Alfred Hitchcock’s famous film was said to be caused by domoic acid poisoning.)

Domoic acid was also implicated by scientists in 2006 when a single sea lion began rampaging in Aquatic Park, accosting 14 swimmers—the largest recent instance of pinniped carnage—around the same time that sea lion attacks were reported in Santa Barbara.

That said, levels of the acid weren’t particularly high last year. Researchers at the Marine Mammal Center treated at least 104 sea lions for domoic acid toxicity in 2017—high by the standards of most years, but well below the 250 or so the center typically treats during a major algal bloom. Rather than acute poisoning, scientists believe that the recent attacks may be the result of longterm, chronic exposure to the acid. If that’s the case, the consequences could be dire: Domoic acid isn’t going away, and the damage to sea lions’ brains, and effects on aggression, could ratchet up, permanently marring our relationship with the tourist icons. For now, those fears have been allayed. Five days after closing Aquatic Park, authorities reopened it to swimmers, with the recommendation to steer far clear of the sea lions. Most humans returned at once, including Einfeldt, who was back in the water the day his doctor OK’d him to swim again. “I was getting saltwater fever,” he says.

As for the sea lions? Einfeldt, a personal injury lawyer, is keeping his misfortune in perspective. “Really, your chances of getting hurt in the water are much lower than getting hurt driving a car.”